Imagine if 9/11 had been really big. Not the c. 2,700 people who are now believed to have died (about the same as die on US roads every month), but larger by a factor of 100 — and in a country with a population a fraction of that of the United States.

That's the scenario of the new political thriller The Ides of March, which premiered last night at the White Bear Theatre in South London. It begins on the one-year anniversary of the day fundamentalist Islamic terrorists exploded a Russian atomic bomb in the Australian city of Melbourne, the date that gives the play its title.

This is a society shaken totally to its foundations, with a bureaucracy struggling to cope and a new secret police force desperate to ensure that nothing like this could happen again — both enjoying almost rabid public support. Set against these forces is a small core of human rights activists and a Muslim community struggling not to be stereotyped as scapegoats. It is much like the United States after 9/11 and Britain after 7/7 in fact.

During the first 15 minutes or so of this show there's cause to fear that you're going to be trapped in a polemical debate about the issues. You meet the chief representatives of each "side": Dr Laura Hammond (Robyn Moore or Eastenders' fame), the sociologist delivering well-honed rhetorical soundbites about freedom — "disunity is a fundamental freedom"; and Warren Hammond (Andrew William Robb) the well-meaning if slightly Alastair Campbell-ish bureaucrat trying to keep the state going. Then there are what might be called the innocent victims: the Muslim father and son (Fanos and Hameem Ziyad), trying to survive as a family in a suddenly overtly hostile land; and the caring father Noel le Bon (Oliver O'Brien) caring for his emotionally scarred daughter Jodie (Joni O'Brien).

Writer Duncan Ley could be accused of telling, not showing, as he sets the scene, and the neat shaping of sides and victims could be overly symmetrical. But once the initial exposition is over with, the script soon takes a more complex, active shape, and the characters, in this strong production with solid performances from actors with a significant range of Shakespearean experience between them, take on truly human, sympathetic complexity.

Indeed, so powerful is the mourning scene, in the tiny, intimate space of the White Bear that I'd have to recommend that anyone who has suffered a recent bereavement not see this play — the raw emotions displayed on stage could be truly disturbing.

The characters very soon stop appearing as symbols of their cause and show both their sympathetic and rougher sides. Indeed if there was to be a structural criticism of this play it would be that each character is almost exactly the same shade of medium grey — the same mix of white good intentions and black hate. There is an exception with the innocent eight-year-old Jodie — a character played very skillfully by the adult Joni after some initial excessive bobby-socks swinging — yet you do wonder if this complication is necessary; she might have worked just as well as an off-stage presence.

Ley does, however, deserve praise for his careful attention to the language of the play. The full colourful possibilities of the Australian form of English are fully explored and offer most of the much-needed lighter moments: "He comes across like a squinty-eyed weasel trying to hide a hard-on" being one of the more memorable insults.

But without a doubt it was the fast-moving, gripping plot that held the first night audience spellbound, and the political complexity of its underlying debate that left them talking passionately in the bar afterwards. Together these elements came together to create a powerful show, one that could easily hold its own in a larger venue.